The area which is covered by a cellular telephony system is divided into so-called cells. In each cell there is a base-station, which handles all communication to and from all cellular telephones in the cell. When a cellular telephone crosses the border between two cells, control of the cell phone's calls is transferred to the base-station of the new cell. In order to facilitate this transfer, so-called hand-off, it is of great interest to know the speed at which the cellular phone is moving.
In, for example, urban areas and other areas with a high call density, it might be desirable to make the cells smaller, since smaller cells will lead to an increase in the amount of base-stations within the area in question. The reason for wanting to increase the amount of base-stations is that this will increase the amount of calls which the system is capable of handling within the area in question.
In areas with many small cells and a high concentration of base-stations, there will be an especially large need to handle hand-off in an optimal manner. In such areas, it will in other words be especially important to know the speed with which the cellular telephone is moving. It is, for example, possible to make a decision not to transfer control of the calls of a cellular telephone to the base-station of a cell if it is known that the cellular phone due to a high speed of motion will not be in that cell for an extended period of time.
The radio signal which is received from the cellular telephone can be attenuated due to multipath propagation, so-called fading. The kind of fading which is practically always present in urban areas is so-called Rayleigh-fading. Rayleigh-fading causes periodical attenuations in the received signal, with the distance in time between the attenuations being dependent on the speed at which the cellular telephone is moving.
The connection between Rayleigh-fading and the speed of motion of the cellular telephone in other words makes it possible to determine the speed at which the cellular phone is moving by looking at the frequency with which the signal received from the cellular telephone falls below a certain level.
Canadian patent CA 2 056 914 discloses a device which works according to the above-mentioned principle. A problem with this device is that it seems to be suited for an ideal system, in which the received signal is strong and without noise. However, in cellular telephony systems, the received signal is often weak and contains a great deal of interference. The interference can be either other radio transmissions or receiver interference. This interference can cause "false" level crossings. Using the device of CA 2 056 914, it can thus be difficult to determine the speed of motion of a cellular phone with useful significance.